vrepl: fix output errors (fix #15801) (#15804)

cmd/tools/vrepl.v

@@ -494,7 +494,9 @@ fn run_repl(workdir string, vrepl_prefix string) int {
 			}
 			if s.output.len > r.last_output.len {
 				len := r.last_output.len
-				r.last_output = s.output.clone()
+				if s.exit_code == 0 {
+					r.last_output = s.output.clone()
+				}
 				cur_line_output := s.output[len..]
 				print_output(cur_line_output)
 			}

vlib/v/tests/repl/error_and_continue_print.repl

@@ -0,0 +1,15 @@
+a = 3
+b
+[4, 5].filter(it < 5)
+===output===
+error: undefined ident: `a` (use `:=` to declare a variable)
+    5 | import math
+    6 | 
+    7 | a = 3
+      | ^
+error: undefined ident: `b`
+    5 | import math
+    6 | 
+    7 | println(b)
+      |         ^
+[4]